The image of the would-be assassin at Donald Trump’s rally in Butler, Pennsylvania, on July 13 is now part of history. A young man in beige lying dead on the flat roof. Minutes earlier, this self-appointed executioner had been pointing his rifle at Trump, aiming to shoot the former president in front of his followers.

What drove this young man to try to kill? Thomas Matthew Crooks was 20 years old, two years out of school and still living with his parents in a town an hour away from the shooting. In his 2022 school yearbook photos, Crooks bears little resemblance to the assassins of film and television who are typically hardened, self-reliant executioners or highly professional hitmen or women.

As far his marksmanship goes, former classmates said Crooks had been rejected from the school rifle team because he was a terrible shot. But coldblooded killers come in all shapes and sizes, as I discovered when I interviewed a number of killers from the streets of Belfast for my 2005 book, Protestant Boy.

These killers, caught up in the Troubles in Northern Ireland, belonged to various paramilitary organisations. They had organisational and social support, and ideologies that allowed them to justify their acts – even the murders of completely........

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